That's the problem here. It costs spell points. Spell points are a limited resource. As the meta is playing with might heroes (because they are obviously way stronger than magic heroes), you have to manage your spell points during the game. Using 2-4 spell points doesn't seem that much when you tell it like that.
Necromancy is a special skill tree available exclusively for Necropolis heroes. The best and most useful skills in the game can be found on necromancy tree. Below skills mostly focus on resurrecting fallen units. Because of that, even after a hard battle you will still have large numbers of creatures that will be added to your army.
Cages: +3 kennels production - a must have. Castle Gate: Town Portal among Inferno Cities with Gates. Great, yet costly! Order of Fire: +1 Spell Power for every visiting hero. Not bad. Resource Silo: one mercury/day. Get it ASAP. Strategy Unless you get +10 ore, you'll be forced to one of those cheaper variants, but don't worry, they're not
Necropolis has at least three very good heroes which will make playing much easier, not only during the start of the adventure. Giovanni with his specialization is one of better heroes. His skill adds bonus for flanking enemy, in some cases it might have a key importance for the outcome of a battle.
8KoCa.